This invention relates generally to the field of preparing silicon substrate wafers for use in the formation of semiconductor devices such as power discrete or power integrated circuits.
Semiconductor devices are built either into polished or epitaxial silicon wafers. The latter consists of an epitaxial (epi) layer on top of a polished wafer substrate. Epi layers typically contain low concentrations of electrically active dopants, usually phosphorous (n-type conductivity) or boron (p-type conductivity), typically close to 1015 atoms cmxe2x88x923. Substrates in many cases contain high concentrations of dopant atoms, which may be phosphorous, antimony, or arsenic (n-type) or boron (p-type), typically in the range 1018-1019 atoms cmxe2x88x923. Dopant levels close to the solubility limit for respective dopant species are needed to lower the resistivity of epi wafer substrates, an important requirement for state-of-the-art power device applications. Silicon material containing such high levels of n-dopant is generally called n+ material. Such material, cut in slices from respective n+ crystals, is used for preparing n+ substrates for ultimate n/n+ epi wafers.
Oxygen is incorporated into growing crystals applying the Czochralski (CZ) technique through dissolution of the fused silica or quartz (SiO2) crucibles used for holding the silicon melt. The molten silicon reacts with the SiO2 crucible wall to form SiO. Some of the SiO evaporates from the melt at the temperature and pressure commonly used for silicon crystal growth. However, some remains in the melt and may be incorporated into the growing crystal. As the melt is solidified, the contact area between the melt and the crucible wall decreases while the area of melt surface available for evaporation of SiO remains substantially constant until near the end of the crystal growth. Consequently, the concentration of oxygen in the melt and therefore the concentration incorporated into the crystal tends to decrease with increasing distance from the seed end of the crystal. Without any countermeasures, this leads to an axial oxygen profile which typically displays decreasing oxygen concentration toward the tail-end of the crystal. The presence of high concentrations of n-type dopants in the silicon melt enhances evaporation of SiO during crystal growing and thereby further reduces the amount of oxygen incorporated into a growing n+ crystal, leading to an axial oxygen profile decreasing heavily toward the tail-end of such a crystal. Without any state-of-the-art countermeasures, after reaching a certain percentage of the total length of such a CZ n+ crystal, the oxygen concentration typically drops below the level required to generate adequate oxygen precipitation when such material is later processed in thermal device manufacturing steps. The length of the crystal at which the oxygen level drops below that required for adequate oxygen precipitation is called the critical crystal length abbreviated Lc.
Oxygen precipitation in epi wafer substrates is the prerequisite for internal gettering (IG) typically applied for controlling the degradation of device yield by way of heavy metal contamination during the thermal device manufacturing steps. Such degradation is described in an article by A. Borghesi, B. Pivac, A. Sassella and A. Stella entitled Oxygen Precipitation in Silicon, published in the Journal of Applied Physics, Vol. 77, No. 9, May 1, 1995, pp. 4169-4244, at 4206-07. Effective IG has been observed at oxygen precipitation related bulk defect densities in the order of 109 atoms cmxe2x88x923. This bulk defect density is critical for effective IG and is referred to hereinafter as the critical defect number Nc. Epitaxial n/n+ wafers based on such high defect density n+ substrates exhibit superior IG related leakage resistance and thereby potentially improved device yield. Thermally induced oxygen precipitation during device processing is suppressed in the case of n-type dopant atoms in epi wafer substrates which creates the necessity to introduce large quantities of oxygen into a crystal. It has been determined experimentally by the inventors hereof that CZ crystals with arsenic concentrations in the order to 1019 atoms cmxe2x88x923 need approximately 8xc3x971017 atoms cmxe2x88x923 oxygen (ASTM 121-83 calibration) in order to reach the Nc necessary for effective IG. Without any state of the art countermeasures, Lc is less than 10% of the total crystal length in this case. In order to essentially increase Lc, effort heretofore has been generally directed at reducing the axial variation of oxygen incorporation. Currently used techniques aiming at axially homogenizing the oxygen level include adjusting crystal pull speed and utilizing crystal and crucible rotation, all in conjunction with controlling gas flow and pressure in the puller chamber. Another technique is the application of defined magnetic fields during crystal growth. These countermeasures are technically sophisticated and/or associated with high cost.
The presence of carbon in silicon wafers has long been known to enhance the precipitation of oxygen. For example, Ahlgren et al. European Application No. 84109528.4 at page 7, lines 26 to 33 teaches that silicon with carbon concentration in excess of 4 ppma (2xc3x971017 atoms cmxe2x88x923) (ASTM 123-76 calibration) can induce substantial oxygen precipitation in silicon containing less than 28 ppma (1.4xc3x971018 atoms cmxe2x88x923) oxygen (ASTM 121-79 calibration) after a thermal treatment that would induce negligible oxygen precipitation at lower concentrations of carbon. It appears that that work refers to the addition of carbon by the usual means as set forth above. Thus, the work accepts the carbon which is introduced as a necessary xe2x80x9cevilxe2x80x9d in consequence of the available equipment used in 1984 and sampling the carbon content along the length of the grown crystal to determine what portion can be advantageously used. Such carbon introduction is uncontrolled and mainly due to the graphite parts used in the puller construction. In current state of the art crystal pullers it is possible to maintain carbon at levels below 5xc3x971015 atoms cmxe2x88x923 in spite of the use of graphite heaters and insulation. Moreover, the European application makes no mention of the presence of n-type or p-type doping materials and it is directed to lightly doped silicon crystals for substrates.
Developments aimed at reducing carbon contamination in crystal growth were originally driven by experimental evidence of detrimental device impact of carbon if present in certain concentration levels within critical device regions of wafers. In the case of epi wafer substrates it is highly unlikely that carbon would enter a critical device regions (typically located in epi layers deposited on top of a substrate) because carbon is a slow diffuser in silicon. Even so, current epi wafer specifications typically still call for carbon concentrations below 1016 atoms cmxe2x88x923.
The present invention is directed to a process for growing silicon crystals wherein predetermined amounts of carbon are added in a controlled fashion to produce the level of oxygen precipitation desired. This process can be effective in n+ doped silicon epi substrates at carbon levels significantly lower than 2xc3x971017 atoms cmxe2x88x923. Rapidly increasing carbon concentration is observed only toward the tail-end of carbon co-doped crystals because its incorporation into the crystal is controlled by the segregation behavior.
Such carbon doping of CZ silicon at a very low concentration can strongly increase the oxygen precipitation in heavily n-doped materials. Moreover, there is a relationship between co-doped carbon, oxygen concentration and bulk defect density after annealing, enabling predetermination of the amount of carbon to be added to achieve the bulk defect level necessary for effective internal gettering. The established methodology allows development of simple and lowcost crystal growing processes leading to enhanced n-type silicon material for epitaxial wafer substrates.